This invention relates to lithographic printing plates, particularly those in which the imaged silver or silver halide areas formed by photographic processes are employed as ink-receptive areas. It relates also to a method of treating such lithographic printing plates.
The lithographic printing plate consists of oleophilic image areas which are receptive to greasy ink and oleophobic non-image areas which are ink repellent. The non-image areas generally consist of hydrophilic areas which are receptive to water. Accordingly, common lithographic printing is performed by supplying both ink and water to the printing plate, whereby the image areas and the non-image areas selectively accept the chromatic ink and water, respectively, and then transferring the ink on the image areas to a substrate such as, for example, paper. Therefore, in order to obtain good print copies it is necessary that there is a sufficient difference in the surface behavior of image areas and non-image areas with respect to oleophilicity and hydrophilicity so that when an ink and water are supplied to the printing plate surface, the image areas may accept the ink in sufficient quantities while the non-image areas may repel the ink completely. Retention of such a difference over a long period of time by the printing plate after its preparation is important also from the commercial viewpoint.
In the graphic art, there have been known various materials for lithographic printing, methods of production thereof and methods of conversion thereof to printing plates. For instance, there are printing plates comprising diazo-sensitized organic colloids supported on metallic plates such as aluminum (PS plates), printing plates utilizing electrophotography, and printing plates involving silver salt photographic processes. These printing plates are now in actual use combined with suitable processing and printing techniques such as oil-desensitizing, dampening, etc., suitable for each type of printing plate.
Photographic elements comprising silver halide emulsion layers having high photosensitivity and capable of being spectrally sensitized are suitable for the automatic production of printing plates and are already in actual use in several forms.
Typical known methods for making lithographic printing plates by utilizing photosensitive gelatino silver halide emulsions may be broadly classified into the following groups:
(1) methods which involve tanning development of hydrophilic gelatino silver halide emulsions to harden the gelatin in the developed areas, thus making it oleophilic and receptive to greasy printing ink (U.S. Pat. No. 3,146,104);
(2) methods which utilize the silver diffusion transfer process to make the metallic silver pattern formed on the surface oleophilic and ink receptive [U.S. Pat. Nos. 3,721,559 and 3,490,905; Japanese Patent Post-examined Publication 30,562/73; A. Rott and L. Dehaes, J. Phot. Sci., Vol. 8, pp. 26-32 (1960)];
(3) methods utilizing the etching bleach process, which comprise treating with a bleach solution the silver image areas formed on development or transfer development, whereby the gelatin in the silver pattern area is destroyed at the same time to expose the oleophilic layer (U.S. Pat. Nos. 3,385,701 and 3,814,603; Japanese Patent Post-examined Publication 27,242/69); and
(4) methods in which undeveloped silver halide image areas in the developed hydrophilic gelatin-silver halide emulsion layer are selectively rendered oleophilic and thus ink receptive (U.S. Pat. Nos. 3,454,398, 3,764,323 and 3,099,209; Japanese Patent Application 84,164/76).
In the methods of (1), the hardened gelatin layer on the surface of printing plate is rendered ink receptive, whereas in the methods of (2) and (4), generally the silver or silver halide on the surface of lithographic printing plate is utilized as ink receptive areas, said silver or silver halide, in most of the cases, being subjected to some treatment or other such as, for example, with a mercapto compound as disclosed in Japanese Patent Post-examined Publications 29,723/73 and 486/76 to enhance oleophilicity, that is, ink receptivity or to convert into ink receptive.
There is a persistent demand for further improvement in printing characteristics such as ink receptivity, anti-staining property, and printing endurance of any lithographic printing plate, not excepting the above-said one which utilizes as ink receptive area the silver or silver halide image formed by photographic processes.